Sensor system for controlling ventilation systems in vehicles

ABSTRACT

A motor vehicle ventilation system having a recirculation or air input mode in which when the rate of change of a signal from an outdoor gas pollution sensor exceeds a threshold limit, the system is adjusted to the recirculation mode.

TECHNICAL FIELD

The invention centers on a sensor system which controls the ventilationsystems in vehicles, both for incoming air and recirculation mode,depending on the concentration of poisonous fumes in the atmosphereoutside the vehicle, using a gas sensor element, the electricalresistance of which falls in case of reducing gases and which rises whenoxidizing gases are present, together with an evaluation unit, theoutput of which is connected to the controls of the ventilation system.

BACKGROUND ART

It is known that by using gas sensor elements, for example tin dioxideelements, the presence of oxidable gases can be registered, for examplethe presence of carbon monoxide, hydrocarbons and hydrogen. During theprocess, a gas-sensitive layer within the gas sensor element, which canconsist of conductive and heated tin dioxide, might be reduced by theoxidable gas; thus a reduction of the ohmic resistance of thegas-sensitive tin dioxide layer in the gas sensor element occurs. Suchgas sensor elements are included in sensor systems which controlventilation systems in vehicles. Whenever the vehicle in question entersan area with a high concentration of poisonous fumes, the ventilationsystem will be switched to recirculation operation, ensuring that thefumes are kept out of the interior of the vehicle.

The usual sensor systems, especially those fitted with tin dioxidesensor elements, react to a very limited extent only to diesel exhaustemissions, although diesel emissions disturb vehicle passengerssubjectively more than many petrol emissions, such as carbon monoxide,since these fumes may not be smelled as obviously as diesel emissionfumes. The reason for this behavior lies in a masking effect whichoccurs, because diesel emissions contain not only oxidable gases, butalso high levels of nitric oxide, which are gases which may be reducedand are thus oxidizing gases. The nitric oxide contained in the dieselemissions once again oxidises the gas sensitive tin dioxide layer of thegas sensor element, which has been reduced by the oxidable gasescontained in the diesel emissions, so that the reducing and oxidizingeffects created by the charging of the gas sensitive tin dioxide layercancel each other out to a great extent.

For an explanation of the `Masking Effect`, see FIGS. 1-3. FIG. 1 showsa sensor circuit, which demonstrates a heated gas sensor element 1 andan external resistor 2. The external resistor 2 can also be replaced bya constant electrical source. The divider current UM measured betweenthe heated gas sensor element 1 and the external resistor 2 is afunction of the gas-altered ohmic resistance of heated gas sensorelement 1.

FIG. 2 shows the change in the ohmic resistance in gas sensor element 1,when coated only with reducing gases or only with oxidizing gases.Impulse 3 shown in the figure is the result of gas sensor elementcontact with gases that can be oxidized, for example with carbonmonoxide (CO). Impulse 4 is the result of gas sensor element 1 contactwith gases that can be reduced, for example with nitric oxide (NOx).

FIG. 3 shows the `Masking Effect`, impulse 5 occurs through gas sensorelement contact with reducing carbon monoxide that can be oxidized. InFIG. 3, impulse 5 is followed by impulse 6, which is achieved by amixture of carbon monoxide and nitric oxide. In spite of the presence ofnitric oxide in the mixture, the ohmic resistance of the gas sensorelement remains lower than in the uncoated state. In spite of thepresence of reducible and thus oxidizing nitric oxides in diesel exhaustemissions, the ohmic resistance of gas sensor element 1 remains underthe level that it would have in an emission-free state. The effectoccurs because of the oxidable and thus reducing gases which are presentin diesel emissions. Under extreme conditions, the ohmic resistance ofthe gas sensor element 1 can show the same level following contact withmixed gas that would be reached by gas sensor element contact to freshair. In any case, the value of the gas sensor signal given followingcontact by gas sensor element 1 with mixed gas is considerably reduced,since in the extreme case demonstrated above the gas sensor signal doesnot differentiate between contact by gas sensor element 1 with mixed gasand gas sensor element contact with fresh air.

Since simply detected oxidable and reducing gases lower the ohmic levelof the tin dioxide layer of the gas sensor element, and reducible andoxidizing gases raise the ohmic level of the tin dioxide layer of thegas sensor element, it had always been assumed that in order to ensureaccurate control of ventilation systems in vehicles that two gas sensorelements would be required. In this way, one gas sensor element servesto measure gases that can be oxidized and are thus reducing, whilst theother gas sensor element serves to detect reducible and thus oxidizinggases.

Sensor systems for controlling vehicle ventilation systems which areequipped with two gas sensor elements are comparatively complicated.This is primarily due to the fact that two gas sensor elements arepresent. Furthermore, the gas sensor signals produced by both of thesegas sensor elements have to be processed and then linked to each otherin such a way as to allow evaluation, which means a comparativelycomplicated organization of the processing and evaluation unit.

DISCLOSURE OF INVENTION

The invention results from the task of developing a sensor system tocontrol ventilation systems in vehicles, both for incoming air andrecirculation more, connected to the concentration of hazardous fumes inthe atmosphere outside the vehicle, through which the sensor tasking andthe evaluation thereof may be carried out more simply and in a lesscomplicated manner.

The task for the sensor system which was explained above has been solvedby the invention of a sensor system that is so organized that, theincrease of a gas sensor signal fed into the evaluation unit followingan increase in the concentration of reducing gases, which amountsapproximately to the fall of the gas sensor signals fed into theevaluation unit for an appropriate increase in the concentration ofoxidizing gases. The system also ensures that the evaluation unit canmeasure the rise and fall of the gas sensor signals per unit of time,and that the evaluation unit transmits a switching signal to alter theoperation of the ventilation system to recirculation mode as soon as thecalculated rise and or fall of the gas sensor signals per time unitexceeds a predetermined limit.

The organization of the sensor system uses the fact that within normalventilation systems for vehicles impulsive occurrence of air pollutionthrough various substances must be expected, and that static conditionsin the atmosphere around the vehicle will not be found, since thevehicle itself will be moving. Furthermore, the sensor system envisagedby the invention uses the typical reaction of a gas sensor element, thatis to say that the gas sensor element coated with a gas typically reactsrelatively quickly in the adsorption phase, whereby the reverse phase,during which the gas sensor element once again reaches its originalstate, takes three to five times longer. The presence of a gasconcentration in the atmosphere around the vehicle will be recognizedand the ventilation system of the vehicle will be switched torecirculation mode, when the rise or fall of the gas sensor elementsignals per time unit exceeds a predetermined limit. It is not importantwhether the gas sensor signal rises or falls, since the degree of therise or fall will be measured and compared to the predetermined limit.Should the limit be exceeded, a switch signal will be generated, toswitch the ventilation system into recirculation mode.

In order to be certain that a switch signal will only be generated toswitch the ventilation system into recirculation mode when a measurabledegree of pollution is present in the air, and in order to avoidunnecessarily frequent switching of the ventilation system, it isadvantageous to ensure that the switch signal is only then generatedwhen the calculated rise and or fall of the gas sensor signal comparedto the limit, actually takes place over a predeterminable period oftime.

In order to ensure that the air inside the vehicle retains a certainquality despite a slow build-up of pollution, as opposed to suddenconcentrations of fumes, it is of advantage for the evaluation unit tocalculate a mean value for the gas sensor signal, measured over a givenperiod of time, whereby the mean value is added to, or subtracted froman absolute value and then placed within a defined limit band, so thatthe switch signal is produced when the gas sensor signal lies outsidethe limits of the band.

The switching behavior of the ventilation system intended by theinvention is feasible with comparatively little complication, if theevaluation unit has a band range which only allows a frequency band topass which indicates a rising or falling value for the gas sensor signalper time unit that exceeds the given limit, over which the gas sensorsignal is conducted, in order to detect such a rise and or fall of thegas sensor signal per time unit, the signal of which will be used toproduce the switch signal to control the ventilation system.

Furthermore, the switch function required by the invention is alsofeasible if the evaluation unit is fitted with a computer to carry out aFourier transformation, whereby the gas sensor signals are examined todiscover whether or not a certain frequency band is present, indicatingthat the limit has been exceeded by rising or falling gas sensor signalsper time unit, with the switch signal for the control of the ventilationsystem being generated in the evaluation unit should such a frequencyband be detected in the gas sensor signal.

A further advantageous example of the invention becomes apparent whenthe evaluation unit is fitted with an electronic neural network, inwhich the gas sensor signal is examined for logical characteristics tosee if a rise or fall in the gas sensor signal per time unit isexceeding the set limit, whereby a switch signal will be generated inthe evaluation unit, should a value from a rise or fall in the gassensor signal that exceeds the set limits be detected during theexamination.

It is possible that the electronic neural network be constructed in theform of a triple or multiple-layer forward coupled neural network. Thusnot only current, but also previously measured gas sensor signals (orsignals calculated from these gas sensor signals) could be used as aninput value. The output of this neural network provides the necessarysignals to control the ventilation system.

As an alternative to a triple or multiple-layer forward coupled neuralnetwork, a triple or multiple-layer backward coupled neural network maybe fitted.

Furthermore, it is possible to describe the switch behavior required bythe invention in logical rules. The evaluation unit can then be fittedwith an electronic fuzzy logic unit, which can be measured by means ofthe rise or fall of the gas sensor signal per time unit which exceedsthe present limit, whereby the switch signal is generated in theevaluation unit when the rise or fall of the gas sensor signal exceedsthe preset limit in the course of the examination.

For practical purposes the evaluation unit is so designed that theswitch signal stops when the gas sensor signal lies inside the handformed by the constructed mean values and no amplitude spectrum isdetected in the rise or fall of the gas sensor signal which exceeds thepreset limits. In this way, it may he ensured that the ventilationsystem of the vehicle in question always functions in recirculation modewhen the level of the air pollution in the atmosphere around the vehicleis unacceptably high and when an unacceptably steep rise in pollution inthe air around the vehicle is detected.

It is extremely important that the return of the switch signal to stopthe operation of recirculation mode, or a switch signal to restart inputof air be generated at a defined point in time, from which time on thequality of the air outside the vehicle may be considered acceptable. Inthis respect, several methods have been suggested, not all of which leadto acceptable results. As an example for the current state of the arttechnology, the method should he described with which the size of thegas contact to a gas sensor element is the value for the operationalperiod of the ventilation system in recirculation mode. Here it isunfavorable that due to a short-term pollution of the air, that may befor example produced at a traffic light, a comparatively longoperational period of the ventilation system in recirculation modefollows, which is completely unnecessary, especially when the vehicle inquestion overtakes the source of the pollution, for example anothervehicle, following the stop at the traffic light. On the other hand, inhighly polluted tunnels, the situation described above leads to theventilation system being switched to recirculation mode due to the highlevel of pollutants in the atmosphere, whereby the system may come tothe end of the set operational period, dependent on the air around thevehicle, and could under certain circumstances switch the system back toair input in the middle of the tunnel. In this way the interior of thevehicle will be supplied with a high level of polluted air.

In order to avoid such a reaction by the ventilation system, theevaluation unit has been fitted with a data storage facility in whichthe peaks of the gas sensor signals are stored, whereby in output theevaluation unit deactivates the switch signal if the gas sensor signaldoes not show a certain predeterminable signal difference from one peakto the next and the gas sensor signal is within the hand for the formedmean value.

It is of utmost importance for the practical requirements of theinvented sensor system to control vehicle ventilation systems that thereactive sensitivity of the sensor system be adjusted to the drivingsituation and the given environment. At this point the capability of thehuman nose should be taken into account, since it too is capable ofadjusting itself to changing pollutant concentrations in the atmosphere.The invented sensor system should thus react extremely sensitively todetected pollutants in areas with mostly unpolluted clean air, whilstthe reactive sensitivity of the invented sensor system must be reducedwhen the vehicle is mostly being driven in areas where the pollutantlevel of the outside atmosphere and the frequency of peak levels ofharmful substances is very high. Such an alteration of the reactivesensitivity is absolutely necessary to achieve a relation betweenventilator operational time in recirculation mode and ventilatoroperational time in air input mode, in order that, even under drivingconditions where the outside atmosphere is highly polluted, theventilation system runs for no more than 50% of the given time inrecirculation mode. Only in this way can the reliable removal ofpassenger-produced moisture and scents from the vehicle interior beensured. This method of operation also avoids the occurrence of fallingoxygen levels within the vehicle which could be caused by an extendedperiod of ventilator operation in recirculation mode, and which couldalso be damaging to passenger health. Assuming that the interior of thevehicle has a volume of 2 cubic meters and that the vehicle is fullyoccupied, a rule applies that the oxygen content in the air may not fallbelow 20%; thus if the occupants of the vehicle are breathing normally,the ventilation system has a maximum operating period of 15 minutes inrecirculation mode. Without an automatic alteration in the reactivesensitivity dependent on the current traffic situation, the operation ofa sensor system is not always satisfactory. A further influence. Stemsfrom the fact that the gas sensor elements in sensor systems candemonstrate a wide range of tolerances with respect to their specificsensitivity, within the range +/-3. Furthermore, gas sensor elements canalter their sensitivity following exposure to certain gaseous substanceswithin their operating life-span. The exposure conditions for gas sensorelements can also be altered by the construction components positionednear to the gas sensor elements. It should also always be taken intoaccount that atmospheric pollution may be divided into base impurities,which may for example be foremost in a town or region, and the occurrentpeak pollutants, produced by vehicles preceding the vehicle in question.The base pollution will normally be lower in rural or suburban areasthan in heavy traffic zones in inner city urban areas.

To compensate for the influences listed above, and to compensate fortheir unwanted effects on the gas sensor element, it is suggested in oneadvantage version of the invention that the evaluation unit should havea regulator to alter the reactive sensitivity of the sensor system,under which the reactive sensitivity of the sensor system may beincreased or decreased according to whether the frequency of the gassensor signals which exceed the preset limit measured over a period oftime is rising or falling, whereby the number of alterations in thereactive sensitivity of the sensor system per unit of time is limited.

Such a variability of the reactive sensitivity of the sensor system canbe set up relatively simply, if the evaluation unit is fitted with atime measurement unit for measuring the operational periods of theventilation system in air input and recirculation mode, a computer unitfor calculating the quotient of operational periods of the ventilationsystem in air input and recirculation mode, and a regulator to reducethe reactive sensitivity of the sensor system when recirculation modeoperation increases and to increase the reactive sensitivity of thesensor system when air input operation is increased, whereby the numberof alterations in the reactive sensitivity of the sensor system islimited over a given period of time.

In a further advantageous model of the invention with variable reactivesensitivity, the regulator unit is fitted with outside temperaturesensor, (which measures the temperature outside the vehicle), aninternal temperature sensor, (which measures the temperature inside thevehicle), together with a comparison unit which compares the outside airtemperature, measured by the outside temperature sensor unit, with theinternal air temperature, as measured by the internal air sensor unit,whereby the regulator increases or decreases the reactive sensitivity ofthe sensor system when the outside air temperature is higher or lowerthan the internal air temperature.

The evaluation unit with the task of controlling the ventilation systemcan take the form of a centrally-programmed microprocessor or beanalogically technical.

The gas sensor element used in the invented sensor system can be a metaldioxide gas sensor element. It should be taken into account that theusual tin dioxide gas sensor elements are so constructed that theirreactive sensitivity towards oxidable gas is particularly high, whilsttheir reactive sensitivity to reducible gases is very low. To be able touse such a metal dioxide gas sensor element within the framework of theinvented sensor system, it must be so fitted that it is more or lessequally sensitive to both oxidable and reducible gases. With most of thecurrent state of the art technology sensor systems, this means thatthese sensor systems are especially susceptible to the type of maskingeffect already described above. On the other hand, metal dioxide gassensor elements which are equally sensitive to both oxidable andreducible gases can be used advantageously in the case of the inventedsensor system, since in this case the degree of rise and or fall of thegas sensor signal is numerically measured.

Whilst fitting the gas sensor elements for the invented sensor system itbecame obvious that basically nearly all current state of the arttechnology metal oxide gas sensor elements, including tin dioxide gassensor elements, reduce their electrical resistance when coated withoxidable gases, and increase their electrical resistance when coatedwith reducible gases. The relation required by the invented sensorsystem with regard to sensitivity cannot be provided by current state ofthe art technology metal oxide gas sensor elements and tin dioxide gassensor elements, since when the gas sensor element comes into contactwith diesel emission fumes the amounts range from 100 to 200 ppb (partsper billion) and by contact with petrol emission fumes from 1 to 50 ppm(parts per million).

For application in the invented gas sensor system, the gas sensorelement is constructed as a mixed oxide sensor element, thegas-sensitive layer consists of tin dioxide (SnO2), tungsten trioxide(WO3), ferric oxide (Fe2O3), aluminium oxide (Al2O3) with platinum (Pt)and palladium (Pd) as reaction accelerators.

The mixed oxide sensor element has particularly advantageouscharacteristics when the gas-sensitive layer has the followingproportions; 29-49%, preferable approximately 39% tin dioxide (SnO2),7-13%, preferably 10% ferric oxide (Fe2O3), 28-48%, preferably 38%tungsten trioxide (Wo3), 7-13%, preferably 10% aluminum oxide (Al2O3),1-3%, preferably 2% palladium (Pd), and 0.5-1.5%, preferably 1% platinum(Pt). It should however be noted that other combinations are possible,by means of which a comparable reactive sensitivity may be reached forcontact with diesel or petrol emission fumes.

The gas sensor element can be mounted directly on an electrical circuitboard and connected with the circuit board by means of precious metalwires.

In one advantageous version of the system the gas sensor element iscontacted with platinum wire and mounted in an opening of an electricalcircuit board of the evaluation unit at approximately the same level,with gas sensor element connections soldered directly onto the circuitboard, so that the gas sensor element is freely mounted.

Metal oxide gas sensor elements alter their characteristics when thetemperature on the surface sensor element changes. When used in thesensor system, the metal oxide gas sensor elements and therefore thesurface of the sensor elements are exposed to the external atmosphere.This situation produces the problem that high speed driving may causethe direct air contact on the surface of the sensor element to create astrong cooling effect.

The suggested solution to the problem was to fit the metal oxide gassensor element with a labyrinth, which would be so designed as to allowthe outside air to reach the metal oxide gas sensor element, but at thesame time to sufficiently protect the metal oxide gas sensor elementagainst moisture, impurities, dirt particles and air currents.

The disadvantage of such a construction would be that, because of thelong way needed for the outside air to travel through the labyrinth, thereactive sensitivity speed of the metal oxide gas sensor element wouldbe slowed down and that various gases would adsorb on the comparativelylarge surface area of the labyrinth.

The solution to the problem was found in the current state of the arttechnology, namely to place the gas sensor element behind gas-permeablebarriers, whereby the barriers allow the gas unlimited access to themetal oxide gas sensor element, whilst at the same time ensuring thatthe sensor has enough protection against moisture, impurities, dirtparticles and air currents.

Such gas-permeable barriers are normally in the form of mountedmembranes, which as long as they are subject to air movements may bebrought to vibrate in this way air movements may be created in the areadirectly around metal oxide gas sensor element, leading to an alterationof the temperature on the surface of the sensor element. The metal oxidegas sensor element reacts to such changes in its surface temperature byaltering its electrical resistance, so that the gas sensor signal isunacceptably overlaid by this distortion factor.

In order to avoid such distortion factors which may render evaluation ofthe gas sensor signal more difficult, the invention version of thesensor system calls for the gas sensor element to be fitted in agas-tight chamber which is in turn fitted opposite the evaluation unit.The chamber has a low volume, is of stable form and has gas-permeablewalls. In this way the required rapid reactive sensitivity of the gassensor element can be reached, since on the one hand the volume of thechamber in which the gas sensor element is placed is as limited aspossible, whilst on the other hand unwanted air movements within thechamber will not occur, due to the stable formation of the walls.

The walls of the chamber may be partially at least constructed ofperforated material, which can hold and mechanically support at leastone layer of the gas-permeable plastic without deformation. It isadditionally possible to place at least one layer of the gas-permeableplastic between two layers of metal webbing.

Instead of placing the gas-permeable plastic between metal webbing, itmay be bedded between two stable sections of perforated thermoplasticmaterial.

It is of advantage if at least one layer of the gas-permeable plastic isconstructed of plastic film.

As long as the plastic film is constructed of teflon or some similarsubstance, then it can be ensured that gas, for example carbon monoxideor nitric oxide, diffuses through the plastic film due to gas pressuredifference, without an actual air transport having taken place. Oneother alternative form would be feasible, whereby the stablegas-permeable walls of the chamber housing the gas sensor element areformed of sintered plastic, glass, metal or similar materials. Suchsintered forms, according to their material, possess a surface thatmakes them more or less watertight, but still allow the passage ofgases.

It is additionally of advantage for the reactive sensitivity of the gassensor element if the chamber housing the gas sensor element ishemispherical.

The fitting of the gas sensor element in a chamber with thecharacteristics described above means that the reaction time of the gassensor element to a change in the pollutant level in the outsideatmosphere is also minimized as a result of the fact that the spacesurrounding the gas sensor element is also minimized. Thus the gassensor element is also well protected against air movements, dust,moisture, water, wax and other aerosols which are used in the motorindustry and maintenance operations. Furthermore, the use of such achamber form allows a low cost, comparatively inexpensive construction.

Many of the application forms of the invented sensor system can benefitfrom having the gas sensor element and the evaluation unit in one singlehousing.

If in this case a form is selected, by which the gas sensor element anda part unit of the evaluation unit are combined in one sensor module,the part unit of the evaluation unit has an oscillating circuit, acapacitor to measure the frequency of the oscillating circuit and aheating regulator to control the temperature of the gas sensor element,and the oscillation circuit and the heating regulator are linked to acentral heating and ventilation control system for the vehicle. Sincethe further processing of an output signal by the sensor module takesplace digitally in the microcomputer of the central heating andventilation control system, it is possible that the gas sensor element,including the directly linked parts of the evaluation unit, whichcorresponds to minimal electronic processing, be fitted in a very smallhousing. The actual processing of the data which comes from the sensormodule then takes place in the microcomputer in the heating andventilation system of the vehicle, which is already present anyway. Byusing this form of the sensor module it may be ensured that the analoggas sensor signal will then be changed into a digital signal within thesensor module be fed in as a digital signal of the actual evaluationwhich takes place in the microcomputer of the vehicle's central heatingand ventilation system.

Analog signals liable to interference thus do not need to be transportedand no complicated analog/digital transformation is necessary within themicrocomputer of the central heating and ventilation control unit.

In order to ensure that the reactive sensitivity of metal oxide gassensor elements in the invented sensor system remains within a positiverange with regard to diesel and petrol emission fumes, it is ofadvantage if the gas sensor element is brought to an operatingtemperature of less than 250 degrees centigrade, with the operatingtemperature being held constant by a heating unit.

One disadvantage in the case of previous sensor systems is the fact thatthe gas sensor elements used in them show major tolerances in theirproduction. A correct electrical impedance match is however only seen ofthe electrical resistance of the gas sensor element and its externalresistance match each other, and the gas sensor current reaches half ofthe operating current.

Apart from these production tolerances, there are also difficulties withthe changing resistance of the gas sensor element which occur as theelement gets older, causing problems with reliable and lastingproduction of impedance match. Furthermore, other sensor systems arenegatively influenced by natural moisture which is picked up by workingmaterials, by certain operating conditions and when in contact withcertain gases. By connection of d.c. voltage water is removed throughhydrolysis. Ion formation takes place and to a high level if iontransport through the crystal mesh of the gas-sensitive layer of the gassensor element.

The listed above can bring about major alterations in thecharacteristics of the gas sensor element.

In order to avoid such effects, it is suggested that the ohmicresistance of the gas sensor element be included in an oscillationcircuit and should be under medium frequency a.c. voltage.

The frequency of the oscillation circuit will preferably be produced bymeans of a capacitor.

One particularly advantageous form of the sensor system has a timer unitfor the oscillation circuit which is switched with a frequency-fixingcapacitor and has s feedback branch, in which the resistance of the gassensor element and s second resistor are fitted, and which shows aparallel branch to the resistance of the gas sensor element, in which athird resistor is fitted. In this way, the minimum and maximum frequencycan be held within limits, even during dramatic changes in theresistance of the gas sensor element. The relation of the gas sensorelement resistor, of the in-line switched resistor, and of the oneparallel-switched to the former resistor, are so selected that even atextremely low ohmic levels, or extremely high ohmic levels of the gassensor element or its ohmic resistance, so that the group resistance ofthe three resistors always remains inside the easily calculated field,which is directly related to the gas sensor signal. Thus it may beensured that the frequency of the gas sensor signals can never push intoa field is outside the recording area of the evaluation electronics ofthe gas sensor element.

BRIEF DESCRIPTION OF DRAWINGS

The various models of the invention are explained below, referring toillustrations.

The following illustrations show:

FIG. 1 shows a prior art circuit.

FIG. 2 shows the change in resistance when 1 is coated only withreducing gases.

FIG. 3 shows the change in resistance when 1 is in contact with areducing gas.

FIG. 4. The process of a gas sensor signal from the invented sensorsystem;

FIG. 5. The process of the gas sensor signal during road surveying workin traffic;

FIG. 6. A band which serves as switch criterion for the invented sensorsystem;

FIG. 7. A further method of creating switch criteria for the inventedsensor system;

FIG. 8. A typical evaluation unit;

FIG. 9. A further model of the evaluation unit for the invented sensorsystem;

FIG. 10. A model of a gas sensor element fitted in a chamber;

FIG. 11. A model of the gas sensor element chamber wall as in FIG. 10;

FIG. 12. A top view of the auxiliary circuit board with gas sensorelement shown in FIG. 10;

FIG. 13. A side projection of the auxiliary circuit board scale 1:1 inFIG. 12;

FIG. 14. The principles of a gas sensor element;

FIG. 15. The principles of one of the primary models of the gas sensorelement from the invented sensor system;

FIG. 16. The principles of a further model of the gas sensor elementfrom the invented sensor system;

FIG. 17. The principles of a circuit for the separation of basicpollution from dynamically occurring peak pollution levels;

FIG. 18. to FIG. 21. Principles of signal processing circuits for theregulation of reactive sensitivity of the invented sensor system;

FIG. 22. A representation of load recognition line for a resistancecapacitor circuit;

FIG. 23. The principles of a sensor module for the invented sensorsystem.

BEST MODE FOR CARRYING OUT THE INVENTION

The conditions which have to be taken into account during theconstruction of the invented sensor system can be derived from theprocess of the gas sensor signal illustrated in FIG. 4. The gas sensorsignal in FIG. 4 is shown as the resistance value of an ohmic resistor Rover time t. At first the gas sensor signal takes on the value 7, whichis brought about when the external air is polluted neither withoxidizing gases nor with reducing gases. This value 7 will be left witha large negative rise should oxidable gases appear, as can be seen inthe process of the gas sensor signal in phase 8. Here the resistance ofthe gas sensor element will have a lower ohmic level, whereby theincrease correlates to the difference between resistance change andelapsing time. Contact of the gas sensor element with oxidable gases,which are especially prevalent in petrol emission fumes, is followed bya de-adsorption phase 9, during which the gas sensor element is movedthrough unpolluted outside air and the gas sensor element returns to itsoriginal states before contact. This de-adsorption phase lasts untilthat point in time where the gas sensor signal has once again reachedits value 7 for unpolluted air. During the de-adsorption phase 9, thepositive increase in the gas sensor signal is much less than thenegative increase of the gas sensor signal during the adsorption phasenamed phase 8. The relation of the resistance change per unit of time isnoticeably smaller during the de-adsorption phase 9 than during theadsorption phase 8.

Once the gas sensor signal has once again taken on its value 7 forunpolluted external air, the gas sensor element runs through an area inwhich the external air is loaded with reducible gases. Because of thecontact by the gas sensor element with these reducible gases, anadsorption phase 10 takes place, during which the resistance value ofthe gas sensor element is greatly increased due to the oxidizing effectof the reducible gases. The positive increase of the gas sensor signalduring adsorption phase 10 are approximately equivalent to the negativeincrease of the gas sensor element during adsorption phase 8. Once thearea containing the air loaded with reducible gases has been passedthrough, de-adsorption phase 11 begins, during which the gas-sensitivelayer of the gas sensor element de-adsorbs. The de-adsorption phasetakes place in the unpolluted outer air, until the gas sensor signal hasonce again adopted the original value 7. The negative increase whicharises from the change in the resistance value per unit of time amountsduring de-adsorption phase 11 to the numeric equivalent of the positiveincrease during the preceding de-adsorption phase 9. It may be observedfrom FIG. 4 that the change in resistance per unit of time during thede-adsorption phases 9 and 11 is noticeably lower than during theadsorption phases 8 and 10.

The increase of the gas sensor signal is negative during the adsorptionphase of a gas sensor element in contact with oxidable gases and duringthe de-adsorption 11 of a gas sensor element in contact with reduciblegases; the increase is positive during de-adsorption phase 9 of a gassensor element in contact with oxidable gases and during adsorptionphase 10 of a gas sensor element in contact with reducible gases.

Characteristic for the process of the gas sensor signal is thedifference between the numerically variant increase values duringadsorption phases 8 and 10, and the de-adsorption phases 9 and 11. Thischaracteristic of the gas sensor signal is used in the case of thisinvention in order to set up objective switch criteria.

When a fall in the gas sensor signal together with a change in the gassensor signal per unit of time that exceeds a preset limit is noted, thegas sensor element has been in contact with oxidable gas. When a rise inthe gas sensor signal together with a change in the gas sensor signalper unit of time that exceeds a preset limit is noted, then the gassensor element has been in contact with reducible gas. The above rulesare valid for every change in the gas sensor signal, independent of thedirection of change of the gas sensor signal, and also independent ofthe starting level of the gas sensor signal at the point in time atwhich an appropriate increase or fall is measured.

Normal road traffic movements do not usually cause situations underwhich the gas sensor element can return to its original conditionfollowing every contact with reducible or oxidable gases and theadsorption phase which results, followed by a de-adsorption phase, whichoccurs with contact to unpolluted air. Rather more, contact withreducible or oxidable gases normally takes place during adsorption andde-adsorption phases which have already started due to other contacts. Atypical process for a gas sensor signal is illustrated in FIG. 5. Thereciprocal value of the resistance value for the gas sensor element isshown over time.

At first the gas sensor element is in contact with oxidable gases whichoriginate from petrol exhaust emissions, from which the adsorption phase12 with its large increase is derived. The end of adsorption phase 12,which occurs before the gas-sensitive layer of the gas sensor element issaturated with oxidable gases, will be followed by de-adsorption phase13. During de-adsorption phase 13 the gas-sensitive layer of the gassensor element de-adsorbs, whereby the de-adsorption speed is noticeablyslower than the adsorption speed during adsorption phase 12. From thisit may be noted that the negative increase in the gas sensor signalduring de-adsorption phase 13 is measurably lower than the positiveincrease in the gas sensor signal during adsorption phase 12. Duringde-adsorption phase 13 the gas sensor element is in contact withreducible gases, for example NO and NOx, as contained in diesel emissionfumes. Following this is an adsorption phase 14 with a large increase,whereby the increase of adsorption phase 14 is numerically equivalent tothe preceding adsorption phase 12. With regard to the process of the gassensor signal, the adsorption phase 14 differentiates itself fromde-adsorption phase 13 insofar as that increase of adsorption phase 14is noticeably greater than the increase during de-adsorption phase 13.

The end of adsorption phase 14, which occurs before the gas-sensitivelayer of the gas sensor element is saturated with oxidizable gases, willbe followed by de-adsorption phase 15, a light coating of the gas sensorelement with oxidable gases and resulting small adsorption phase 16,immediately followed by a short term coating of the gas sensor elementwith large quantities of reducible gases which could originate from theexhaust system of a lorry, and the resulting adsorption phase 17, ade-adsorption phase 18, a renewed coating with reducible gases whichleads to adsorption phase 19, a de-adsorption phase 20, a coating withoxidable gases, which leads to adsorption phase 21, a coating withreducible gases, which leads to adsorption phase 22, a de-adsorptionphase 23, a coating of the gas sensor element with oxidable gases, whichleads to adsorption phase 24, a de-adsorption phase 25, a renewedcoating of the gas sensor element with oxidable gases, which leads toadsorption phase 26, a coating of the gas sensor element with reduciblegases, which leads to adsorption phase 27, a short de-adsorption phase28, a renewed coating of the gas sensor element with reducing gases,which leads to adsorption phase 29, and a de-adsorption phase 30.

From the representation of the gas sensor signal in FIG. 5 it canclearly be seen that the increase in the adsorption phases isnumerically clearly greater than the increase in the de-adsorptionphases. The principle of the invention is based on setting a lower limitvalue for the increase of the gas sensor signal, with the result that aswitching of the vehicle ventilation system from air input torecirculation mode will always occur when the increase in the gas sensorsignal exceeds this limit value.

With the switching behavior of the ventilation system stated above, theinput into the vehicle of the suddenly polluted external air can beavoided. This switching behavior is however not suitable for preventingthe air quality inside the vehicle from being affected by a situationwhere a gradual increase in pollutants in the air surrounding a vehicle,which may occur in tunnels or other heavy traffic situations, is takingplace.

In order to also prevent such prevailing quality deteriorations in theair surrounding the vehicle which also lower the quality of theatmosphere within the vehicle, a value with a mean of 31 is calculatedfrom the gas sensor signal by integration or similar means, valid for aset period of time, as shown in FIG. 6. A band 32 is constructed aroundthis mean value 31, which can take place through subtraction andaddition of an absolute or by supplement or deduction basedproportionally on the value 31. This band 32 allotted to the mean value31 is limited by the upper value 32 and by the lower limit 34, wherebythe upper value 33 and the lower value 34 mark the positive/negativeproportional or absolutely permitted deviation of the gas sensor signalfrom the mean value 31.

A switch signal to switch the vehicle ventilation system from air inputmode to recirculation mode will then be generated whenever the currentgas sensor signal lies outside the field of band 32 and thus inside thefields 35 or 36.

The signal to switch the vehicle ventilation system from air input modeto recirculation mode must be generated at a moment in time from whenthe quality of the air surrounding the vehicle is of an acceptablestandard. In order to achieve such switching behavior, recirculationmode will be activated when,--as can be seen in FIG. 7, at time point 37a fall starts in the gas sensor signal, whereby the negative increase ofthis fall in the gas sensor signal is greater than the reciprocal valuementioned above. Before time point 37 the gas sensor signal runs at itsnormal value 38 which it has in unpolluted atmospheric conditions.During adsorption phase 39, following time point 37, the gas sensorsignal falls to inversion point 40, at which the de-adsorption phase 41begins, following adsorption phase 39. At inversion point 40 the signfor the increase in the gas sensor signal changes from a negative signduring adsorption phase 39 to a positive sign during de-adsorption phase41. Through this sign change for the increase of the gas sensor signalinversion point 40 is recognized and stored in the evaluation unit.Furthermore, the gas sensor signal at the time of inversion of the signof the gas sensor signal will be linked to this inverse point 40. Afreely determinable difference value 42 is stored in the evaluationunit. If the gas sensor signal level during de-adsorption phase 41leading from inversion point 40 increases by the difference value 42,there follows a switching of the vehicle ventilation system fromrecirculation mode to air input mode. Before the switching operation isinitiated, the evaluation unit checks to see whether the gas sensorsignal is actually within the band 32 illustrated in FIG. 6.

At time point 37 the gas-sensitive layer of the gas sensor element iscoated with an oxidizable gas. After the completion of de-adsorptionphase 41, the gas-sensitive layer of the gas sensor element is againcoated with a reducible gas. At the beginning of this contact at timepoint 43, the increase of the gas sensor signal changes from thecomparatively low value during the de-adsorption phase 41 to therelatively high value during the adsorption phase 44, which takes placefollowing time point 43. The increase in the gas sensor signal duringadsorption phase 44 exceeds the preset limit value, so that a switchingof the vehicle ventilation system from air input to recirculation modetakes place at time point 43. Adsorption phase 44 leads to de-adsorption46 after the coating of the gas-sensitive layer in the gas sensorelement by reducible gases at inversion point 45 has ended. The sign forthe increase of the gas sensor signal changes at inversion point 45,between adsorption phase 44 and de-adsorption phase 46. Through thischange of sign, inversion point 45 will be recognized and stored in theevaluation unit. In addition, gas sensor signal level 47 is stored ininversion point 45 in the evaluation unit. Furthermore, a differencevalue 48 is stored in the evaluation unit and may be freely allotted.The difference value 48 for de-adsorption phase 46, following a coatingwith reducible gases, can be the equivalent of the difference value 42for de-adsorption phase 41 following coating with oxidable gases, butmay nevertheless be different since diesel and petrol exhaust emissionsare not sensed in the same way.

If the gas sensor signal, leading from inversion point 45, duringde-adsorption phase 46 falls below gas sensor signal level 47 atinversion point 45 by the difference value 48, then the vehicleventilation switches from recirculation mode to air input operation.

Graph 49 shows the operational state of the ventilation system. Fromgraph 49 it may be seen that the ventilation system worked inrecirculation mode until switching into air input mode, from tim point37 following an increase by difference value 42 in the de-adsorptionphase 41 leading from inversion point 40 and between time point 43 andthe fall by difference value 48 in de-adsorption phase 46 which followsinversion point 45.

The basic construction of the sensor system invention is illustrated inFIG. 8.

A sensor part 50 and an evaluation unit 51 belong to the sensor system.

Sensor part 50 has a gas sensor element 52, which has an changeableohmic resistance and is held by means of a heating unit 53 at anoperating temperature of less than 250 degrees centigrade. Furthermore,sensor part 50 has an external resistor 54. A gas sensor signal measuredbetween gas sensor element 52 and external resistor 54 is fed into theevaluation unit through a connection cable 55. This central evaluationunit 51 serves not only to evaluate the gas sensor signal data itreceives through the connection cable 55, but also to control thevehicle ventilation system which is not shown in FIG. 8.

The evaluation unit 51 is fitted with an integrated analog/digitaltransformer 56, which changes the analog gas sensor signal fed into theevaluation unit 51 into a digital signal. Additionally, a programme runis digitally stored in the evaluation unit 51, by means of which aswitch signal may be generated using the switch criteria explained inFIGS. 4-7, switching the vehicle ventilation system from air input torecirculation mode and vice-versa.

In the model of the sensor system which is illustrated in FIG. 9, thesensor part 50 is the same as sensor part 50 described in FIG. 8 above.The gas sensor signal travels from sensor part 50 through the connectioncable 55 into evaluation unit 51.

In evaluation unit 51 the gas sensor signal travels through a band pass57. Band pass 57 is so arranged that only those amplitude spectrums canpass which are typical for a process of the gas sensor signal, when thegas sensor signal changes in line with an adsorption phase occurring ingas sensor element 52. This means that band pass 57 can only allow thoseamplitude spectrums to pass, whose gas sensor signal increases aresignalled as exceeding the preset threshold. In this respect it shouldonce again be said that gas sensor element 52 is so set up (in a waydescribed below) that the increase during its contact with oxidablegases and the resulting adsorption phase is the same as the increaseduring coating with reducible gases and the resulting adsorption phase.

The start signal of band pass 57 is rectified in demodulator 58. Therectified start signal from demodulator 58 triggers a comparator with afreely selected shifter shaft if the level is correct. In this manner, aswitch signal for the ventilation system will always be generated whenthe vehicle fitted with the invented sensor system drives through areaswhere the outside atmosphere is polluted, whereby the sensor system'sswitching behavior is independent of whether or not the level ofimpurities in the air derive from diesel or petrol emissions.

This switch signal remains at the output of comparator 59 as long as thegas sensor element 52 of sensor part 50 is in the adsorption phase.Saturation of gas sensor element 52 almost never occurs in practise,since the vast majority of the impurities found in the outsideatmosphere are of relatively short term nature, due to the fact that thevehicle will move through the atmosphere during the course of normaloperation. Adsorption phases last a few minutes at the most, so that gassensor element 52 will normally not be held in a saturated state.

There are however traffic situations in which a saturation of gas sensorelement 52 cannot always be avoided. The placing of gas sensor element52 in the saturation zone may for example occur when the vehicle remainsfor a longer period of time in an area where the outside atmosphere isheavily polluted. In such a case, the increase in the gas sensor signalwill move towards zero, although the pollution of the outside atmosphereremains unchanged or may even deteriorate. Without further measures thevehicle ventilation system would, in such a traffic situation, switchrecirculation mode to air input mode, since the gas sensor signal insuch a traffic situation would now show the frequency bands that couldtravel through band pass 57.

Since such situations may occur for example in traffic tunnels and heavytraffic jams, the actual level of the gas sensor signal will be measuredand processed in the evaluation unit. In this way, the gas sensor signalfed into the evaluation unit 51 through the connection cable 55 is fedinto an resistance capacitor circuit, by means of which it may beintegrated with a high time constant. Accordingly, a mean value for thegas sensor signal forms in a capacitor 61 of the resistance capacitorcircuit. Whenever the gas sensor signal takes on a higher value than themean value which is in the capacitor 61 of the resistance capacitorcircuit 60, current builds up in load resistor 62 in the resistancecapacitor circuit, which is the equivalent of the difference betweencurrent gas sensor signal and the mean value of the gas sensor signalwhich is in capacitor 61 of the resistance capacitor circuit. Thiscurrent is fed into a comparator 63, by which means a switch signal willalways be generated, if the current gas sensor signal is greater thanthe quasi `stored` mean value in capacitor 61 of the resistancecapacitor circuit 60.

By correct selection of the time constant of the resistance capacitorcircuit 60 in the traffic situations described above, where theatmosphere surrounding the vehicle is subject to lasting high pollutionlevels and thus the gas sensor element 52 can be saturated, it can beensured that the vehicle ventilation system can remain in recirculation.

Even when the vehicle is surrounded by very high levels of pollution fora prolonged period of time, it is still desirable that the vehicleventilation system should once again switch from recirculation mode toair input mode after a predetermined period of time; otherwise theoxygen consumption by the vehicle passengers, the moisture and otherphysiological factors deriving from those in the vehicle would rapidlylead to a fall in the air quality inside the vehicle. In order to avoidsuch a situation arising, the time point is stored in the evaluationsystem, at which the vehicle ventilation system switches from air inputto recirculation mode. At the end of a preset period of time, the lengthof which is dependent on the volume of the inside of the vehicle, aswell as the measured number of persons travelling in the vehicle, thevehicle ventilation system will be forcibly switched from recirculationmode to air input mode, as long as, within the time space in question,no switch from recirculation mode to air input has taken place due to analteration in the level of the pollution in the atmosphere surroundingthe vehicle. The output signal from comparator 59 and the output signalfrom comparator 63 of evaluation unit 51 will be linked together inlinking unit 64. The output signal from linking unit 64 will be fed intothe controller of the ventilation system through output cable 65 oflinking unit 64 or evaluation unit 51 as a switching signal.

In a preferable model of the invented sensor system the circuitfunctions explained above using analog function groups will be computerconstructed. This then allows a microprocessor controlled processallowing, according to the desired switch behavior explained above,switch signals for the switching of the vehicle ventilation system fromair input to recirculation mode, or vice-versa, to be received.

Gas sensor element 52 is preferably constructed as a mixed oxide sensorelement, whereby its gas-sensitive layer contains approximately 39% tindioxide, approximately 10% ferric oxide, approximately 38% tungstentrioxide, approximately 10% aluminium oxide, approximately 2% palladiumand approximately 1% platinum, whereby the palladium and the platinumfunction as reaction accelerators.

In a preferred model version of the invented sensor system the gassensor element will be run at an operating temperature that lies under200 degrees centigrade. At such an operating temperature for gas sensorelement 52, its reactive sensitivity towards oxidable gas is greatlyreduced. The reactive sensitivity of gas sensor element 52 towardsreducible gases is however comparable with the reactive sensitivity atother temperatures. In this way it may be ensured that, in view of theirdisturbance potential for the vehicle passengers, comparable coating ofthe gas sensor element 52 with reducible and oxidable gases will produceapproximately the same, even if directionally different, alterations ofthe gas sensor signal. In view of this even-handed sensitivity of thegas sensor element 52 towards oxidizable and reducible gases, it ispossible to use the alteration of the gas sensor signal per unit oftime, that is the figure for the increase in the gas sensor signal, as aswitching parameter for the vehicle ventilation system.

In a further preferred model of the invented sensor system the sensorpart 50, together with evaluation unit 51, are fitted in one commonhousing.

In the model illustrated in FIG. 10 the gas sensor element is mounted ona circuit board 66 and fitted inside chamber 67. Chamber 67 is formed bya wall or screen 68 and by horizontal plate 69. Horizontal plate 69 hasan opening 70 in its cross-section, approximately equivalent to that ofcircuit board 66, through which circuit board 66 stretches into chamber67. Between the inside of opening 70 and that part of circuit board 66which is inside the opening, sealant material 71 is intended. Gas sensorelement 52 is directly mounted here on circuit board 66, whereby circuitboard 66 provides the connection to the evaluation 51, which is notillustrated in FIG. 10. In this model, the evaluation unit 51 is fittedin a separate housing. Chamber 67 and the housing for evaluation unit51, not illustrated in FIG. 10, are hermetically sealed from each otherby means of the horizontal plate 69 and the sealant material 71. Such ahermetical division is required, in order to minimize the decay time forexample of gas sensor element 52 and to exclude any distortion factorswhich may for example derive from the electronic components inevaluation unit 51 warming in the emissions, or warming plastic parts.

The wall/screen 68, which surrounds gas sensor element housing chamber67 is constructed on a gas-permeable basis. In this way, rapid andunhindered gas exchange between chamber 67 and the air surroundingwall/screen 68 can take place.

One constituent part of the wall 68 is a gas-permeable film 72. Teflonhas proven itself to be an especially suitable material for this film.

The film 72, is as can especially be seen in FIG. 11, covered andsupported on both sides by stable wall sections 73 and 74. It is somounted between wall sections 73 and 74 that vibration of film 72 may beexcluded in any case.

Wall sections 73 and 74 can be in the form of drawn form parts of wovenwire or as drawn perforated metal or plastic.

The wall/screen 68 can cup-shaped or hemispherical in form, whereby thestability of the shape may achieved by pressing a teflon film 72 withglass-fibre strengthened plastic to the required wall or screen 68.

In another model the wall or screen 68 used to protect againstmechanical wear can be formed of a gas-permeable sinter body or of agas-permeable plastic body.

One possibility for mounting gas sensor element 52 on circuit board 66is shown in FIGS. 12 and 13, whereby FIG. 13 represents excerpt I--I inFIG. 12.

A rectangular cut-out 75 exists in circuit board 66. Cut-out 75 ispositioned in the free end section of that part of circuit board 66which extends into chamber 67.

Gas sensor element 52 is positioned in the middle part of cut-out 75 incircuit board 66. Gas sensor element 52 is linked to connection cables76, made of platinum or any other suitable precious metal and which areconnected to connection pins 77, fitted in the area surrounding cut-out75 in circuit board 66. The mounting of gas sensor element 52 thus takesplace using connection cables 76 and the circuit board-side connectionpins 76. The connection pins 76 could for example be made of nickel.

The fitting of gas sensor element 52 in cut-out 75 of circuit board 66is so selected that the gas sensor element 52 is positioned in the exactcenter of the cut-out 75 of circuit board 66, as measured in everydimension.

Instead of the sensor part 50 illustrated alone in FIG. 14, which showsthe gas sensor element 52 with ohmic resistance changed according to gascontact, heating unit 53 and outside resistor, whereby the actual gassensor signal is transmitted through connection wire 55 to evaluationunit 51, which is not illustrated, the invented sensor system can makeuse of sensor part 78, which is illustrated in FIG. 15 and describedbelow. In the case of the sensor part 78 illustrated in FIG. 15, a gassensor element 80 which can be heated using a heater unit 79 is planned,the ohmic resistance 81 of which is combined with oscillation circuit82, which has a further frequency fixing component of a capacitor 83.The oscillation circuit 82 produces an output signal, which is fedthrough cable 84 to a further evaluation unit.

In one practically applicable model, as illustrated in FIG. 16, circuit82 is planned as an electronic standard component in the form of, forexample, a timer 555. This timer component is switched with frequencyfixing capacitor 85.

Timer component 82 has a feed-back branch 86. Feed-back branch 86 has anohmic resistance 87 of a gas sensor element 88, adjustable according togas coating, with a parallel-switched resistor 89. A third resistor 90is planned for feed-back branch 86, in series to the sequence of theparallel switched resistor 87 and 89.

Through the parallel switching of the second resistor 89 to resistor 87of gas sensor element 88 and the serial switching of the third resistorto the sequence of the parallel switched resistors 87 and 89, theminimum and maximum frequency will be limited, even in the case of adramatic change in resistor 87 of gas sensor element 88 following suddenand strong gas contact. The relations between the three resistors 87, 89and 90 are so selected that both in case of extremely low ohmic leveland in the case of extremely high ohmic level in resistor 87 of the gassensor element 88, the group resistance of resistors 87, 89 and 90always remain inside an easily calculated range, related directly to thefrequency of the initial signal. In this way it may be ensured that thefrequency of the initial signal or of the gas sensor signal will nevermove into a field which is outside the operating range of the switchedevaluation unit.

FIG. 17 shows a switching system by means of which basic pollution canbe differentiated from peak levels of pollution. Basic pollution of theatmosphere outside the vehicle, such as that which may be experienced ina rural area, is very different to that degree of impulsive, dynamicpeak pollution levels which might well be faced in an inner-city area,such as would be experienced behind a diesel-powered lorry or at aparticularly busy road traffic junction. With this switching system, itis assumed that, based on past experience, basic pollution levels changeextremely slowly, whereas on the other hand the pollution levelsproduced by a vehicle driving in front may change extremely quickly.

A gas sensor element 91 illustrated in FIG. 17 is fed by a constantsource of current 92. The gas sensor current measured between theconstant current source 92 and gas sensor element 91 is fed by means ofa cable 93 to a divider 94.

A cable 95 branches from the cable 93, by means of which the gas sensorsignal will be fed into an integrator 96, by means of which acomparative signal will be generated for an operation amplifier 97. Thegas sensor signal is fed directly into the other input of the operationamplifier 97. The output signal from operation amplifier 97 is also fedto divider 94.

In an output cable 98 of divider 94, an output signal is made available,by means of the fact that changing base pollution means slowly-changingsignal parts of the gas sensor signal depending on the time constant ofthe time unit or integrator 96 are separated from the gas sensor signal.The output signal in the output cable 98 of divider 94 only contains thepart signal from the gas sensor signal which shows the dynamic peakpollution levels.

In FIGS. 18 to 21, further circuit models are illustrated, by means ofwhich the possibility of so processing the gas sensor signal exists thatthe reactive sensitivity of the invented sensor system will becontrolled dependent on those part signals from the gas sensor signalwhich derive from the previously-mentioned dynamic peak pollutionlevels.

In the example illustrated in FIG. 18, the sensor signal is fed througha cable 99 to a diode 100, through which the gas sensor signal 99charges a resistance capacitor circuit; in this way a current will becreated, and fed to a adjustable amplifier 102.

The level of the output signal in the output cable 103 of the adjustableamplifier 102 is therefore dependent of the charge condition of theresistance capacitor circuit 101, which is logically dependent on thedynamics and frequency of the gas sensor signal.

A switch amplifier 104, fitted after the adjustable amplifier, for whichoutput cable 103 of the adjustable amplifier functions as input cable,has a fixed set trigger level which is fed into the switch amplifier 104through input cable 105. The switch amplifier 104 then produces a switchsignal in its output cable 106 whenever the adjusted gas sensor signallevel exceeds the limit preset by means of the trigger level.

As in the case of the model illustrated in FIG. 18, the so-calleddynamic signal works for the model illustrated in FIG. 19. The dynamicsignal contains merely those part signals which derive from dynamiccoating of the gas sensor element, due to impulsive peak pollutionlevels. These part signals are transmitted through a cable 107 to adiode 108, through which a resistance capacitor circuit 109 will becharged. The current produced by means of resistance capacitor circuit109 is fed into a circuit amplifier 111 through a connection link cable110, by means of which amplifier 111 has a signal-dependent and thusvariable trigger current.

Furthermore, the dynamic gas sensor signal is fed into switch amplifier111 through amplifier 112, whereby it triggers the switch amplifier 111,according to the switch threshold, that is to say according to thevariable trigger current produced by the resistance capacitor circuit. Aswitch signal will then be generated in an output cable 113 of switchamplifier 111.

FIG. 20 shows an illustration of a further model, in which the dynamicgas sensor signal, explained above and mentioned on several occasions,travels along cable 114 through an amplifier 115 before being fed intoswitch amplifier 116.

The switch current derives from the current of a current divider withswitched capacitor circuit 117, whereby the current of the output signalin an output cable 120 of switch amplifier 116 through a diode 118 and acharge resistor 119 influences the capacitor circuit 117, with thetendency that, as long as there is frequently output or switch signal inthe output cable 120 of switch amplifier 116, the trigger threshold ofswitch amplifier 116 is so altered that the switch system reacts lesssensitively to changes in the dynamic gas sensor signals travellingthrough cable 114.

In the model illustrated in FIG. 21, in much the same way as the modelillustrated in FIG. 20, the dynamic gas sensor signal travels alongcable 121 through an adjustable amplifier (in this model) 122 beforebeing fed into switch amplifier 123. The output signal for the switchamplifier 123 present in the output cable 124 travels through chargeresistor 125 and diode 126 and thus charges a resistance capacitorcircuit 127 switched after diode 126, so that regulated voltage occurs,which is fed into the adjustable amplifier 122 via connection cable 128,thus influencing the amplification of adjustable amplifier 122.

The switch amplifier 123 which is switched after adjustable amplifier122 works with a fixed set switch threshold.

FIG. 22 shows how output signals 129 from the switch amplifier influencethe charge curve 130 of the resistance capacitor circuit. It is clearlydemonstrated that the charge current is a function of the frequency aswell as of the duration of the switching of the output signal of theabove-mentioned switch amplifier.

As already mentioned above, the evaluation unit with the circuitsdescribed above may be fitted as a centrally-programmed microprocessor,which works according to the above-listed switching criteria as adigital/numerical model.

The model of the invented sensor system illustrated in FIG. 23 is theactual sensor part combined with the minimal amount of evaluationelectronics necessary and fitted in a protective housing. The actualevaluation of data takes place in the microcomputer which is fitted inthe heating and air-conditioning system of many vehicles already.

The model of the invented sensor system illustrated in FIG. 23 is fittedwith a sensor module 131, a gas sensor element 132, oscillation circuit133, a capacitor 134 which determines the frequency of oscillationcircuit 133 and a heater control 135.

Gas sensor element 132 has an alterable (according to gas coating) ohmicresistor 140, connected to oscillation circuit 133, the frequency ofwhich is determined by capacitor 134. By means of heater controller 135the heater unit 141 of the gas sensor element 132 is adjusted.

Sensor module 131 is linked by means of electrical cables 136 and 137 toa voltage source, for example the central heating and ventilationsystem.

The gas sensor signal, altered through the oscillation circuit fitted asa Rf circuit, is fed into microcomputer 139 of the central heating andventilation controller unit via cable 138.

We claim:
 1. Sensor system to control ventilation systems in vehicles inrecirculation or air input mode dependent on the pollution levels in theair outside the vehicle, with a gas sensor element (52) the electricalresistance of which sinks in the presence of reducing gases and whichrises in the presence of oxidizing gases, and an evaluation unit (51),the output of which is connected to the controller of the ventilationunit characterized by the fact that the sensor system is so organizedthat increase of a signal fed into the evaluation unit following an risein the concentration of reducing gases is approximately quatitivelyequivalent to fall of the gas sensor signal fed into the evaluation unit(51) following a rise in the concentration of oxidizing gases, that theevaluation unit (51) calculates the rise or fall per time unit in thegas sensor signals which are fed into it and the evaluation unit (51)generates a switch signal to adjust the ventilation system torecirculation mode as soon as the measured increase or decrease in thegas sensor signal per time unit numerically exceeds a threshold limit.2. Sensor system according to claim 1, under which a mean value (31) iscalculated for a set time period from the gas sensor signal in theevaluation unit (51), this mean value (31) is allotted to a definedlimit band (32) by proportional subtraction and addition of an absoluteor by supplement or deduction based proportionally, and the switchsignal is generated when the gas sensor signal lies outside this definedlimit band (32).
 3. Sensor system according to claims 1 or 2, underwhich the evaluation unit (51) has a band pass (57) which only allows anamplitude spectrum to pass, which displays the threshold limit-exceedingrise or fall of the gas sensor signal per time unit, through which gassensor signal travels, in order to detect such a rise or fall of the gassensor signal per time unit the output signal used to generate theswitch signal for the control of the ventilation system has to be takeninto account.
 4. Sensor system according to claims 1 or 2, under whichthe evaluation unit (51) is equipped with a computer facility to enablethe completion of a Fourier transformation, in which the gas sensorsignal is arithmetically investigated for the presence of a certain riseor fall of the gas sensor signal per time unit displaying amplitudespectrum which exceeds a threshold limit, and in which the switch signalis generated in the evaluation unit in order to control the ventilationsystem, whenever such an amplitude spectrum is detected in the gassensor signal.
 5. Sensor system according to claims 1 or 2, under whichthe evaluation unit is fitted with a electronic neural network, in whichthe gas sensor signal is investigated for characteristics showing thepresence of a threshold-excessive rise or fall in the gas sensor signalper time unit, and under which the switch signal is generated in theevaluation unit (51), if a rise or fall in the gas sensor signal whichexceeds the threshold is detected in the cause of this investigation. 6.Sensor system according to claim 5, under which the electronic neuralnetwork takes the form of a triple or multiple-layer forwards-coupledneural network.
 7. Sensor system according to claim 5, under which theelectronic neural network takes the form of a triple or multiple-layerinverse-coupled neural network.
 8. Sensor system according to claim 1 or2, under which the evaluation unit (51) is fitted with an electronicFUZZY logic unit by means of which a rise or fall in the gas sensorsignal per unit of time which exceeds the threshold value may beregistered, and in which the switch signal is generated in theevaluation unit (51), if a rise or fall in the gas sensor signal perunit of time which exceeds the threshold is detected in the cause ofthis investigation.
 9. Sensor system according to one of claims 1 to 2,in which the evaluation unit (51) is fitted with a storage unit, inwhich the peak points (40, 45) of the gas sensor signal are stored, andin which the switch signal is extinguished on the output side of theevaluation unit, if the gas sensor signal after the peak point (40, 45)shows a certain predeterminable signal level difference (42, 48) to thepeak point (40, 45) and the gas sensor signal lies inside the band (32)allotted to the formed mean value (31).
 10. Sensor system according toone of claims 1 to 2, in which the evaluation unit (51) is fitted with acontroller unit to adjust the reactive sensitivity of the sensor system,by means of which the reactive sensitivity of the sensor system may beraised or lowered in case of rising or falling frequency of increasesand decreases in gas sensor signal which exceed the threshold limitvalues, whereby the numerical degree of the alteration of the reactivesensitivity of the sensor system per unit of time is limited.
 11. Sensorsystem according to claim 10, in which the controller unit used toadjust the reactive sensitivity of the sensor system is fitted with anexternal temperature sensor, which registers the air temperature outsidethe vehicle, an internal temperature sensor, which registers the airtemperature inside the vehicle, and which has a comparative unit, whichcan compare the outside temperature provided by the external temperaturesensor with the internal air temperature provided by the internaltemperature sensor, whereby the controller unit raises or lowers thereactive sensitivity of the sensor system when the outside temperatureis higher or lower than the inside temperature of the vehicle. 12.Sensor system according to one of claims 1 to 2, by which the evaluationunit (51) is fitted with a timer unit, by means of which the operatingperiods of the ventilation system in air input mode and in recirculationmode are measurable, a computer unit, by means of which the quotient ofoperating periods in air input mode and the operating periods of theventilation system in recirculation mode is calculable, and is fittedwith a controller, by means of which the reactive sensitivity of thesensor system may be reduced in case of increased recirculation modeoperation, and increased in case of increased air input operation,whereby the degree of alteration of the reactive sensitivity per unit oftime is limited.
 13. Sensor system according to claim 12, in which thecontroller unit used to adjust the reactive sensitivity of the sensorsystem is fitted with an external temperature sensor, which registersthe air temperature outside the vehicle, an internal temperature sensor,which registers the air temperature inside the vehicle, and which has acomparative unit, which can compare the outside temperature provided bythe external temperature sensor with the internal air temperatureprovided by the internal temperature sensor, whereby the controller unitraises or lowers the reactive sensitivity of the sensor system when theoutside temperature is higher or lower than the inside temperature ofthe vehicle.
 14. Sensor system according to one of claims 1 to 2, inwhich the evaluation (51) unit, together with the controller for theventilation system, is analog technically formed.
 15. Sensor systemaccording to one of the claims 1 to 2, in which the gas sensor element(52) takes the form of a mixed oxide sensor element, the gas-sensitivelayer of which contains tin dioxide (SnO2), tungsten trioxide (WO3),ferric oxide (Fe2O3), aluminium oxide (Al2O3), with platinum (Pt) andpalladium (Pd) as reaction accelerators.
 16. Sensor system according toclaim 15, in which the gas-sensitive layer of the mixed oxide sensorelement (52) takes the form of a mixed oxide sensor element, containing29% to 49% tin dioxide (SnO2), 28% to 48% tungsten trioxide (WO3), 7% to13% ferric oxide (Fe2O3), 7% to 13% aluminium oxide (Al2O3), with 0.5%to 1.5% platinum (Pt) and 1% to 3% palladium (Pd).
 17. Sensor systemaccording to one of claims 1 to 2, in which the gas sensor element (52)is electrically contacted with precious metal wires, preferably platinumwires, and is mounted in a cut-out (75) within an electrical circuitboard (66) to the evaluation unit (51) with approximately the samemiddle plane as the circuit board, and gas sensor element connectionsdirectly soldered to the circuit board (66), so that the gas sensorelement (52) hangs freely.
 18. Sensor system according to one of claims1 to 2, in which the evaluation unit (51) is, together with thecontroller for the ventilation system, in the form of acentrally-programmed microprocessor.
 19. Sensor system according to oneof claims 1 to 2, in which the gas sensor element (52) is placed in achamber (67) which is closed gas-tight to the evaluation unit (51) andhas a low volume, together with stable, gas-permeable walls.
 20. Sensorsystem according to claim 19, in which at least one layer (72) of thegas-permeable material is positioned between two layer (73, 74) of wovenmetal mesh.
 21. Sensor system according to claim 20, in which at leastone layer (72) of the gas-permeable material is made of plastic film.22. Sensor system according to claim 19, in which at least one layer(72) of the gas-permeable material is made of plastic film.
 23. Sensorsystem according to claim 22, in which the plastic film is made ofTeflon or some similar material.
 24. Sensor system according to claim23, in which the stable formed and gas-permeable walls (68) of thechamber (67) housing the gas sensor element (52) is made of sinteredplastic, glass, metal or some similar material.
 25. Sensor systemaccording to claim 19, in which at least one layer (72) of thegas-permeable material is bedded between two stable formed andperforated layers made of thermoplastic material.
 26. Sensor systemaccording to claim 25, in which at least one layer (72) of thegas-permeable material is made of plastic film.
 27. Sensor systemaccording to one of the claims 1 to 2, in which the ohmic resistor (140)for the gas sensor element (132) is part of an oscillation circuit (133)through which flows middle frequency a.c. voltage.
 28. Sensor systemaccording to claim 27, in which a capacitor (134) is fitted, whichproduces the frequency of the oscillation circuit (133).
 29. Sensorsystem according to claim 28, in which the oscillation circuit (82) isfitted with a timer component (82), which is switched with a frequencyfixing capacitor (85) and/has a feed-back branch (86), in which theresistor (87) of the gas sensor element (88) and a second resistor (90)are fitted, and which has a parallel branch to the resistor (87) of thegas sensor element (88), in which a third resistor (89) is fitted. 30.Sensor system according to claim 27, in which the oscillation circuit(82) is fitted with a timer component (82), which is switched with afrequency fixing capacitor (85) and has a feed-back branch (86), inwhich the resistor (87) of the gas sensor element (88) and a secondresistor (90) are fitted, and which has a parallel branch to theresistor (87) of the gas sensor element (88), in which a third resistor(89) is fitted.
 31. Sensor system according to claim 19, in which thewalls (68) of the chamber (67) at least, partially consist of aperforated work material, which holds stably and mechanically supportsat least one layer of a gas-permeable material.
 32. Sensor systemaccording to claim 31, in which at least one layer (72) of thegas-permeable material is bedded between two stable formed andperforated layers made of thermoplastic material.
 33. Sensor systemaccording to claim 32, in which at least one layer (72) of thegas-permeable material is made of plastic film.
 34. Sensor systemaccording to claim 31, in which at least one layer (72) of thegas-permeable material is made of plastic film.
 35. Sensor systemaccording to claim 31, in which at least one layer (72) of thegas-permeable material is positioned between two layer (73, 74) of wovenmetal mesh.
 36. Sensor system according to claim 35, in which at leastone layer (72) of the gas-permeable material is made of plastic film.37. Sensor system according to one of claims 1 to 2, in which the gassensor element (132) and a part unit (133, 134, 135) of the evaluationunit are brought together in a sensor module (131), the part unit (133,134, 135) of the evaluation unit fitted with a oscillation circuit(133), a capacitor (134) to determine the frequency of the oscillationcircuit (133) and a heater controller (135) to control the temperatureof the gas sensor element (132), and the oscillation circuit (133) andthe heater controller (135) are connected to a central heating andventilation controlling device (139) for the vehicle, whereby theprocessing of an output signal from the sensor module (131) takes placecompletely digitally in the microcomputer of the central heating andventilation controlling device (139).
 38. Sensor system according to oneof the claims 1 to 2, in which the gas sensor element made of metaloxide is kept at a constant temperature by means of a heating element(53).
 39. Sensor system according to claim 38, in which the oscillationcircuit (82) is fitted with a timer component (82), which is switchedwith a frequency fixing capacitor (85) and has a feed-back branch (86),in which the resistor (87) of the gas sensor element (88) and a secondresistor (90) are fitted, and which has a parallel branch to theresistor (87) of the gas sensor element (88), in which a third resistor(89) is fitted.